Photoluminescence and photoredox catalytic properties of cationic Ru(ii) polypyridine complexes encapsulated within an InTATB metal–organic framework

Abstract

The nanoscale environment within the void spaces of metal–organic frameworks (MOFs) can significantly influence the photoredox catalytic activity of encapsulated visible-light photoredox catalysts (PCs). To compare two isostructural PC@In-MOF systems, three cationic Ru(II) polypyridine complexes were successfully encapsulated within the mesoscale channels of the anionic framework of InTATB (H₃TATB = 4,4′,4′′-s-triazine-2,4,6-triyltribenzoic acid), which features a doubly interpenetrated framework structure. This encapsulation yielded three heterogenized visible-light PCs, RuL₃@InTATB, where L = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or 2,2′-bipyrazine (bpz). A simple cation-exchange process partially replaced the NEt₄⁺ counter-cations in the as-prepared InTATB with the cationic Ru(II) polypyridine complexes. The TATB³⁻ linker contains a central triazine unit, a strong electron acceptor, which can engage in intermolecular π–π interactions with one of the three ligands of the [RuL₃]²⁺ complexes. This electronic interaction makes InTATB a more favorable MOF host for stabilizing the excited state of Ru(II)* polypyridine complexes, compared to the previously studied isostructural MOF, InBTB (H₃BTB = 1,3,5-benzenetribenzoic acid), which features a central phenyl donor. The photoluminescence (PL) spectra and PL lifetimes of the three RuL₃@InTATB systems were investigated. In all cases, the PL lifetimes were significantly extended compared to those of the corresponding free [RuL₃]²⁺ complexes. The photoredox catalytic activity was evaluated using the aza-Henry reaction of 2-phenyl-1,2,3,4-tetrahydroisoquinoline (THIQ) under visible light at room temperature (RT). The RuL₃@InTATB systems exhibited higher conversion rates and enhanced product selectivity compared to their RuL₃@InBTB counterparts, likely due to their extended PL lifetimes. These heterogenized systems also demonstrated strong catalytic performance in the visible-light-driven aerobic oxidation of benzyl halides in the presence of an auxiliary organocatalyst, also under mild conditions at RT.